3-Methylglutaraldehyde 3-methyl-1,5-pentanedial, hereinafter abbreviated as “MGL”) is a useful compound as a curing gent for photosensitive material, a tanning agent or leather, and a synthesis intermediate (see, for example, PTLs 1 to 3). As a production method of MGL, a method of hydrolyzing pyranyl ether which is obtained by the Diels-Alder reaction between croton aldehyde and methyl vinyl ether is known (see NPLs 1 and 2).